1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a semiconductor device wherein the lower portion of a contact hole is cleaned, and doped with an impurity for improving the refresh characteristics and reducing the contact resistance to improve operation capability of the device.
2. Description of the Background Art
As the integration density of semiconductor device is increased, the line width of the device has decreased. A various processes have been developed and used to fabricate devices with excellent characteristics.
In particular, a contact formation process for reducing contact resistances has been newly suggested to improve operation efficiency of a device.
In a conventional semiconductor device, a silicon contact material consists of polysilicon containing impurities. In case of an ideal contact interface, a contact resistance due to difference of work function does not exist between the polysilicon and the semiconductor substrate. That is, the contact resistance between the silicons is extremely low when they have nearly equal impurity concentrations. However, the contact resistance between the polysilicon and the semiconductor substrate generally is relatively high in real cases. The contact resistance of n-type contact cell having a contact area of 0.10 μm2 is about 10 kΩ. Such a high contact resistance is due to a native oxide film and carbon containing residues at the interface between the polysilicon and the semiconductor substrate.
Generally, even though a polysilicon layer is deposited after a wet cleaning process without any delays in a conventional contact formation process, the contact resistance cannot be effectively reduced.
A conventional method for fabricating a semiconductor device will now be described.
A device isolation film defining an active region is formed on a semiconductor substrate.
A buffer layer having a predetermined thickness is formed on the entire surface of the resulting structure. Preferably, the buffer layer consists of a nitride film, and serves as an etch barrier in a subsequent etching process for forming a contact hole to prevent damages of the active region.
An interlayer insulating film is formed on the buffer layer. Preferably, the interlayer insulating film is an oxide film, and has an etching selectivity over the buffer layer.
The interlayer insulating film and the buffer layer is etched using a contact mask exposing a predetermined contact area to form a contact hole. In the etching process, the interlayer insulating film is removed using the buffer layer as an etch barrier, and the buffer layer is then removed to prevent damage of the semiconductor substrate.
The lower portion of the contact hole is cleaned. The cleaning process of the lower portion of the contact hole comprises a dry cleaning process for removing portions damaged in the contact hole etching process, and a wet cleaning process for removing carbon composite residues and a native oxide film.
Thereafter, an impurity is ion-implanted into the lower portion of the contact hole.
A contact material is formed on the entire surface of the resulting structure to fill the contact hole, and then etched back or planarized by chemical mechanical polishing process to form a contact plug.
As described above, in accordance with the conventional method for fabricating the semiconductor device, the cleaning process using a volatile organic compound solution and deionized water during the wet cleaning process after the formation of the contact hole is performed last, and thus fails to efficiently prevent the formation of a native oxide film and carbon residues on the surface of the semiconductor substrate, which increases the contact resistance.